The present invention relates to a method for correcting the positional deviation of a conveyed item in a conveyance system, in particular a sheet or a material web, and a device for correcting the positional deviation of a conveyed item.
When conveying objects, in particular sheets, material webs or other goods to be conveyed, it is often necessary to hold the conveyed items in a defined position, to correct their position relative to the conveyance system, or to adjust downstream stations, in particular processing stations, to the current position of the conveyed items. To ensure this, the actual position of the conveyed item is usually detected and compared with a target value. If deviations are identified, correction of the position of the conveyed item is initiated, for example. For example, general conveyance procedures in “in-line” conveyance machines, i.e., in cyclical machines with a recurrent production process, corrections of the position of the conveyed item itself, or corrections of a downstream processing station can be carried out in the machine. After a positional deviation is detected, the conveyance procedure is corrected by superimposing a correcting motion on the synchronous machine operation.
A further example of this can be seen in the register control for printing processes in sheetfed printing. In this process, the position of the sheet on impression cylinder 16 can be adjusted such that it has precise register. By maintaining precise register, it can be guaranteed that the impression zones of the different colors are positioned exactly.
For this purpose, ink or another marking applied separately to the printing stock can be scanned. Based on the sampled value determined in this manner, the current actual position value of the printing stock can be determined using a time or location-based method of evaluating the current actual position value of the printing stock, for example. By making a comparison with a target value, a deviation can be identified which must be corrected in order to process the printing stock with positional accuracy in a downstream processing step. Processing steps of this nature can include an additional application of ink, cutting the printing stock, embossing or folding, for example. To perform the correcting motion, a correcting motion can be superimposed on the synchronous machine operation of the downstream processing unit.
As known from EP 753 407 A2, for example, the lateral alignment of a sheet in a printing press can be corrected. To this end, a positional deviation of the side edge of the sheet from the target value is first determined using a target vs. actual comparison of measured value. The sheet is then brought into the target position using a correcting motion.
In performing corrections of this type, the correction path is the path along which the conveyed item moves in the transport direction while the correcting motion is being carried out. The overall, superimposed travel carried out which therefore corresponds to the duration of the correction action, is referred to as the correction. If, in an x-y diagram, the correction path is plotted on the x-axis and the correction is plotted on the y-axis, a graphic representation results which is referred to as the correction profile. In terms of performing corrections, a defined correction profile is permanently specified in machines used today to convey sheets, such as printing presses, post-printing finishing machines such as folding machines or other machines for transporting conveyed items. These permanently specified correction profiles are therefore not adaptable to different applications or even different machine types.